Why do you have to log in? To prove to the system that you are a registered member and therefore have been approved and authorized to post messages on DC. Guests, and members who are not logged in, are "read only" status.
When you log in, click on the little box that says, "Remember Me?" - putting a check in this box will save your login info in a "cookie" that is automatically read when you come to the DC website. It also keeps track of which messages you're read, etc. Although this is a text file - the information is encrypted so nobody can just open it and read your user name or password. If you "log out" this cookie is deleted and you will have to log in again the next time you visit DC.
If you didn't log out, but have to log in every time you visit DC - you have something either preventing you from creating a cookie, or automatically deleting it when you leave DC. This could be caused by settings in your operating system, internet software, or by some security software.